Catalyst for isomerizing alkylaromatics

ABSTRACT

This invention presents a novel catalyst formulation for the isomerization of alkylaromatic hydrocarbons. The catalyst is comprised of at least one Group VIII metal, and a pentasil zeolite having an x-ray diffraction characteristic of ZSM-12 wherein a portion of the aluminum atoms have been replaced with gallium atoms. When utilized in a process for isomerizing a non-equilibrium mixture of xylenes containing ethylbenzene, a greater yield of paraxylene is obtained compared to prior-art processes.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of prior copendingapplication Ser. No. 281,424, filed 12-8-88 now U.S. Pat. No. 4,886,927which is a division of Ser. No. 109,019, filed 10-16-87, abandoned, thecontents of which are incorporated herein by reference thereto.

FIELD OF THE INVENTION

This invention relates to an improved catalyst for the isomerization ofxylenes and conversion of ethylbenzene. More specifically, the inventionconcerns a catalyst composition comprising a Group VII metal componentand a gallium-substituted pentasil zeolite.

BACKGROUND OF THE INVENTION

The xylenes, namely ortho-xylene, meta-xylene and para-xylene, areimportant chemicals and find wide and varied application in industry.Ortho-xylene is a reactant for the production of phthalic anhydride.Meta-xylene is used in the manufacture of plasticizers, azo dyes, woodpreservers, etc. Para-xylene upon oxidation yields terephthalic acidwhich is used in the manufacture of synthetic textile fibers.

As a result of the important applications to which the individual xyleneisomers are subjected, it is often very important to be able to producehigh concentrations of a particular xylene. This can be accomplished byconverting a non-equilibrium mixture of the xylene isomers, whichmixture is low in the desired xylene isomer, to a mixture whichapproaches equilibrium concentrations. Various catalysts and processeshave been devised to accomplish the isomerization process. For example,it is well known in the art that catalysts such as aluminum chloride,boron fluoride, liquid hydrofluoric acid, and mixtures of hydrofluoricacid and boron fluoride can be used to isomerize xylene mixtures.

Industrially, isomerization of xylenes and conversion of ethylbenzene isperformed primarily to produce para-xylene. A typical processing schemefor this objective comprises: (a) separating para-xylene from a C₈alkylaromatic mixture using, for example, molecular sieve technology, toobtain a para-xylene-rich stream and a para-xylene-depleted stream; (b)isomerizing the para-xylene depleted stream to near equilibrium in anisomerization reaction zone; and, (c) recycling the isomerizationproduct to separation along with the fresh C₈ alkylaromatic mixture.

The present invention is particularly concerned with the isomerizationreaction step which may be used in an overall process directed topara-xylene production. An important parameter to consider in thisisomerization reaction step is the degree of approach to xyleneequilibrium achieved. It is desirable to run the isomerization processas close to equilibrium as possible in order to maximize the para-xyleneyield. However, associated with this is a greater cyclic C₈ loss due toside-reactions (cyclic C₈ hydrocarbons include xylenes, ethylbenzene,and C₈ napthenes.) The approach to equilibrium that is used is anoptimized compromise between high C₈ cyclic loss at high conversion(i.e. very close approach to equilibium) and high utility costs due tothe large recycle rate of unconverted ethylbenzene, ortho-xylene,meta-xylene, and C₈ napthenes which result from the hydrogenation of theC₈ aromatics. The correlation of cyclic C₈ loss versus the distance fromxylene equilibrium is a measure of catalyst selectivity. Thus there is astrong incentive to develop a catalyst formulation which minimizescyclic C₈ loss while maximizing para-xylene yield.

Numerous catalysts have been proposed for use in xylene isomerizationprocesses such as mentioned above. More recently, a number of patentshave disclosed the use of crystalline aluminosilicate zeolite-containingcatalysts for isomerization and conversion of C₈ alkylaromatics.Crystalline aluminosilicates generally referred to as zeolites, may berepresented by the empirical formula:

    M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O

in which n is the valence of M which is generally an element Group I orII, in particular, lithium, sodium, potassium, magnesium, calcium,strontium, or barium, and x is generally equal to or greater than 2.Zeolites have skeletal structures which are made up of three-dimensionalnetworks of SiO₄ and AlO₄ tetrahedra, corner-linked to each other byshared oxygen atoms. Zeolites with high SiO₂ /Al₂ O₃ ratios havereceived much attention as components for isomerization catalysts.Representative of zeolites having such light proportion of SiO₂ includemordenite and the ZSM varieties. It is also known in the art thatzeolites of the ZSM series can be prepared with gallium atomssubstituted for aluminum atoms, for example, see U.S. Pat. No.4,585,641. In addition to the zeolite component, certain metal promotersand inorganic oxide matrices have been included in isomerizationcatalyst formulations. Examples of inorganic oxides include silica,alumina, and mixtures thereof. Metal promoters such as Group VIII orGroup III metals of the Periodic Table, have been used to provide adehydrogenation functionality. The acidic function can be supplied bythe inorganic oxide matrix, the zeolite, or both.

When employing catalysts containing zeolites for the isomerization ofalkylaromatics, characteristics such as acid site strength, zeolite porediameter, and zeolite surface area become important parameters toconsider during formulation development. Variation of thesecharacteristics in a way that reduces side-reactions, such astransalkylation, is required in order to achieve acceptable levels ofcyclic C₈ loss.

It has been found that, if a catalyst is formulated with the components,and in the manner set forth hereinafter, an improved process for theconversion of a non-equilibrium mixture of xylenes containingethylbenzene is obtained.

OBJECTS AND EMBODIMENTS

A principal object of the present invention is to provide a novelcatalyst for the isomerization of isomerizable hydrocarbons. Morespecifically, the instant invention is aimed at a catalyst compositionwhich, when utilized for the isomerization of alkylaromatichydrocarbons, results in minimal loss of the alkylaromatic hydrocarbons.Other objects of the instant invention are to present a method ofpreparation and a process use of the catalyst.

Accordingly, a broad embodiment of the invention is directed toward acatalyst for the isomerization of isomerizable hydrocarbons comprisingat least one Group VIII metal component, and a gallium-substitutedpentasil zeolite. A preferred pentasil zeolite having an x-raydiffraction characteristic of ZSM-12.

Another embodiment is directed toward a process for the isomerization ofa feed stream comprising a non-equilibrium mixture of xylenes containingethylbenzene, which comprises contacting the feed in the presence ofhydrogen at a temperature of from about 300° and 500° C., a pressure offrom about 69 to about 6895 kPa(ga), a liquid hourly space velocity offrom about 0.5 to about 10 hr⁻¹ with a catalyst comprising at least oneGroup VIII metal component and a gallium-substituted pentasil zeolitehaving an x-ray diffreaction characteristic of ZSM-12.

These as well as other objects and embodiments will become evident fromthe following more detailed description of the invention.

INFORMATION DISCLOSURE

The prior art recognizes numerous isomerization processes employing avariety of catalyst formulations. However, it is believed that none ofthe prior art processes recognizes the use of the catalyst formulationand method of making same which forms an integral part of the instantinvention.

U.S. Pat. No. 3,923,639 (Ciric) is directed to a hydrocarbon crackingprocess utilizing a catalyst composition comprising a crystallinealuminosilicate ZSM-4 zeolite. Although the reference lists as possiblecomponents Group VIII metals and a variety of matrix materials, thereference is silent as to the utility of a gallium-substituted pentasilin combination with a Group VIII metal for the isomerization ofalkylaromatic hydrocarbons.

The conversion of heavy reformate using a variety of different catalystcompositions, including silica-alumina containing pentasil zeolites, istaught in U.S. Pat. No. 4,066,531 (Owen et al). However, the referenceis not cognizant of the utility of a gallium-substituted pentasilzeolite in combination with the other components of the instantinvention.

U.S. Pat. No. 4,255,288 (Cull et al) teaches a catalyst comprising aY-type zeolite, alumina, zirconia, and at least one each of Group VIBand Group VIII metals. Hydrocracking and hydrodesulfurization tests showsuperior results for catalysts of the invention. The reference does notdisclose a gallium-substituted pentasil zeolite.

Several relevant references are directed to processes and catalystcompositions specifically for isomerizing alkylaromatics. Related U.S.Pat. Nos. 4,331,822 and 4,485,185 (Onodera et al) teach the use of acatalyst containing silica-alumina pentasil zeolites having addedthereto platinum and a second meatl. However, neither referencerecognizes gallium-substituted pentasil zeolites. U.S. Pat. No.4,482,773 (Chu et al) is directed to a process for isomerizing a mixtureof xylenes and ethylbenzene with a ZSM-5 catalyst containing platinumand a Group IIA component. However, the reference does not recognize theutility of gallium substitution in the zeolite. Another reference, U.S.Pat. No. 4,584,423 (Nacamuli et al), teaches a process for isomerizing anon-equilibrium mixture of xylenes containing ethylbenzene in theabsence of hydrogen using a catalyst containing ZSM-5 or ZSM-11 whereingallium may be substitued for aluminum. This reference makes no mentionof the utility of a Group VIII metal.

U.S. Pat. No. 4,599,475 (Kresge et al) teaches an isomerization processusing a catalyst comprising ZSM-23 zeolite. Kresge et al disclosegallium among a broad range of nine "Y" framework elements and aluminaamong a non-limiting list of seven binder materials. However, Kresge etal teach away from the use of a larger-pore zeolite than ZSM-23 in thedisclosed range of framework and binder elements.

In summary, it appears that the prior art only generally recognizes thatzeolites have utility for isomerization of alkylaromatics and that nosingle reference teaches nor suggests the invention claimed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 compares the performance of Catalyst A of the invention andCatalyst B of the prior art, relating ethylbenzene conversion to theparaxylene content of the xylenes product.

FIG. 2 compares the performance of Catalysts A and B of the inventionand prior art, respectively, by relating para-xylene yield to the ratioof C₈ cyclics recycled from the isomerization reactor to the para-xyleneseparation zone.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, this invention is concerned with a catalystcomposition useful for the isomerization and conversion of anon-equilibrium mixture of C₈ aromatic hydrocarbons. This catalyticcomposite comprises at least one Group VIII metal component, and apentasil zeolite having an x-ray diffraction characteristic of ZSM-12wherein a portion of aluminum atoms have been replaced with galliumatoms. When utilized in a process for isomerizing a non-equilibriummixture of alkylaromatics, the instant invention allows for a closerapproach to xylene equilibrium resulting in a greater yield ofpara-xylene without the high loss of C₈ aromatics common to prior artprocesses.

The catalyst of the instant invention contains at least one Group VIIImetal component of the Periodic Table (see, Cotton and Wilkinson,Advanced Inorganic Chemistry (3rd Ed., 1972)). Preferably, this GroupVIII metal is selected from the platinum group metals. Of the platinumgroup metals, which include palladium, rhodium, ruthenium, osmium andiridium, the use of platinum is preferred. The platinum group componentmay exist within the final catalyst composite as a compound such as anoxide, sulfide, halide, oxysulfide, etc., or as an elemental metal or incombination with one or more other ingredients of the catalyst. It isbelieved that the best results are obtained when substantially all theplatinum group component exists in the elemental state. The platinumgroup component generally comprises from about 0.01 to about 2 wt % ofthe final catalytic composite, calculated on a elemental basis. It ispreferred that the platinum content of the catalyst be between about 0.1and 1 wt. %. The preferred platinum group component is platinum, withpalladium being the next preferred metal. The platinum group componentmay be incorporated into the catalyst composite in any suitable mannersuch as by coprecipitation or cogelation with the inorganic-oxidematrix, or by ion-exchange or impregnation of the zeolite, or byion-exchange or impregnation of the zeolite and matrix composite. Thepreferred method of preparing the catalyst normally involves theutilization of a water-soluble, decomposable compound of a platinumgroup metal to impregnate the composite. For example, the platinum groupcomponent may be added to the composite by commingling the compositewith an aqueous solution of chloroplatinic or chloropalladic acid. Anacid such as hydrogen chloride is generally added to the impregnationsolution to aid in the distribution of the platiumu group componentthrough the composite particles.

After addition of the Group VIII metal component to the zeolite andmatrix composite, the resultant composite is dried at a temperatureranging from about 100° to about 200° C. for a period of at least 2 toabout 24 hours or more, and finally calcined or oxidized at atemperature ranging from about 450° to about 650° C. in air or oxygenatmosphere for a period of about 0.5 to about 10 hours in order toconvert all of the metallic components to the corresponding oxide form.The resultant oxidized composite is preferably subject to asubstantially water-free reduction step prior to its use in theisomerization of hydrocarbons. This step is designed to selectivelyreduce the Group VIII metal component to the elemental metallic stateand to ensure a uniform and finely divided dispersion of the metalliccomponent throughout the catalyst. Preferably, a substantially pure anddry hydrogen stream (i.e. less than 20 vol. ppm H₂ O) is used as thereducing agent in this step. The reducing agent is contacted with theoxidized catalyst at conditions including a reduction temperatureranging from about 200° to about 650° C. and a period of time of about0.5 to 10 hours effective to reduce substantially all of the Group VIIImetal component to the elemental metallic state.

The resulting reduced catalyst composite may, in some cases, bebenefically subjected to a presulfiding operation designed toincorporate in the catalytic composite from about 0.05 to about 0.5 wt.% sulfur calculated on an elemental basis. Preferably, this presulfidingtreatment takes place in the presence of hydrogen and a suitablesulfur-containing compound such as hydrogen sulfide, lower molecularweight mercaptans, organic sulfides, etc. Typically, this procedurecomprises treating the reduced catalyst with a sulfiding gas such as amixture of hydrogen and hydrogen sulfide having about 10 moles ofhydrogen per mole of hydrogen sulfide at conditions sufficient to effectthe desired incorporation of sulfur, generally including a temperatureranging from about 10° up to about 593° C. or more. It is generally agood practice to perform this presulfiding step operation undersubstantially water-free conditions.

The gallium-substituted pentasil zeolite utilized in the instantinvention preferably has a formula (expressed in terms of mole ratios ofoxides) as follows:

    M.sub.2/n O:W.sub.2 O.sub.3 :ySiO.sub.2 :zH.sub.2 O

wherein M is at least one cation of valence n, W is gallium and/oraluminum, y is at least 5, preferably at least 12, and z is from 0 to40. The zeolite preferably has an X-ray diffraction characteristic ofpentasil zeolites, which includes ZSM-5, ZSM-8, ZSM-11, ZSM-12, ZSM-23,and ZSM-35, with ZSM-12 being particularly preferred. "Pentasil" is aterm used to describe a class of shape-selective zeolites. This novelclass of zeolites is well known to the art and is typicallycharacterized by a silica/alumina mole ratio of at least about 12.Suitable descriptions of the pentasils may be found in U.S. Pat. Nos.4,159,282; 4,163,018; and 4,278,565, all of which are incorporatedherein by reference. The zeolite framework may contain only gallium andsilicon atoms or may contain a combination of gallium, aluminum, andsilicon atoms. The gallium content, expressed as mole ratios of SiO₂/Ga₂ O₃, may range from 20:1 to 400:1. The preferred gallium-substitutedpentasil zeolite has a ZSM-12 structure with a gallium content rangingfrom 0.1 to 10 wt. % of the zeolite, most preferably ranging from 0.5 to5 wt. %. The gallium-substituted pentasil zeolite may be prepared bycrystallization from a reaction mixture comprising a silica source, asource of Ga₂ O₃, a source of Al₂ O₃ if desired, and optionally anorganic template compound. It is believed that the preparation ofzeolites is within the competence of one skilled in the art and aparticular preparation method is not critical to the instant invention.It is preferred that the catalyst of the instant invention contain from1 to 20 wt. % gallium-substituted ZSM-12 zeolite. In a preferredembodiment, the catalyst comprises an inorganic-oxide matrix.

The inorganic oxide matrix utilized in the present invention preferablyis a porous, adsorptive, high-surface area support having a surface areaof about 25 to about 500 m² /g. The matrix should also be uniform incomposition and relatively refractory to the conditions utilized in thehydrocarbon conversion process. By the term "uniform in composition", itis meant that the support be unlayered, has no concentration gradientsof the species inherent to its composition, and is completelyhomogeneous in composition. Thus, if the support is a mixture of two ormore refractory materials, the relative amounts of these materials willbe constant and uniform throughout the entire support. It is intended toinclude within the scope of the present invention matrix materials knownin the art such as (1) activated carbon, coke, or charcoal; (2) silicaor silica gel, silicon carbide, clays and silicates including thosesynthetically prepared and naturally occurring, which may or may not beacid treated, for example, attapulgus clay, diatomaceous earth, fuller'searth, kaolin, kieselguhr, etc.; (3) ceramics, porcelain, bauxite; (4)refractory inorganic oxides such as alumina, titanium dioxide, zirconiumdioxide, chromium oxide, zinc oxide, magnesia, thoria, boria,silica-alumina, silica-magnesia, chromia-alumina, alumina-boria,silica-zirconia, zirconia-alumina, etc; and (5) combinations of one ormore elements from one or more of these groups. The preferred matricesfor use in the present invention are refractory inorganic oxides, withbest results obtained with a binder comprised of alumina. Suitablealuminas are the crystalline aluminas known as the gamma-, eta-, andtheta-aluminas. Excellent results are obtained with a matrix ofsubstantially pure gamma-alumina. In addition, in some embodiments, thealumina matrix may contain minor proportions of other well knownrefractory inorganic oxides such as silica, zirconia, magnesia, etc. Thepreferred combination of inorganic oxides is zirconia with gamma- oreta-alumina, especially from about 90 to 99 wt. % alumina and from about1 to 10 wt. % zirconia. Matrices preferably have an apparent bulkdensity of about 0.3 to about 0.8 g/cc and surface area characteristicssuch taht the average pore diameter is about 20 to 300 angstroms and thepore volume is about 0.1 to about 1 cc/g. Preferably the matrix isuniform in composition, may be prepared in any suitable manner, and maybe synthetically prepared or naturally occurring. Whichever type ofmatrix is employed, it may be activated prior to use by one or moretreatments including but not limited to drying, calcination, andsteaming.

Using techniques commonly known to those skilled in the art, thecatalyst of the instant invention may be composited and shaped into anyuseful form such as spheres, pills, cakes, extrudates, powders,granules, tablets, etc., and utilized in any desired size. These shapesmay be prepared utilizing any known forming opertions including spraydrying, tabletting, spherizing extrusions, and nodulizing. A preferredshape for the catalyst composite is the extrudate prepared using thewell-known extrusion method. Here the pentasil zeolite with or withoutmetallic components added is combined with the binder and a suitablepeptizing agent and mixed to form a homogeneous dough or thick paste.This material is then extruded through a die pierced with multiple holesand the spaghetti-shaped extrudate is cut off on the opposite side toform short cylinders. The rheological properties of the dough mixturecan be varied by the use of "extrusion aids" such as methylcellulose,stearates, small amounts of clay, colloidal silica, etc. Afterextrusion, the cylinders are dried and calcined as set forthhereinbelow.

An alternative shape of the composite is a sphere, continuouslymanufactured by the well-known oil drop method. Preferably, this methodinvolves dropping the mixture of zeolite, alumina sol, and gelling agentinto an oil bath maintained at elevated temperatures. The droplets ofthe mixture remain in the oil bath until they set and form hydrogelspheres. The spheres are then continuously withdrawn from the oil bathand typically subjected to specific aging treatments in oil and anammoniacal solution to further improve their physical characteristics.

The resulting aged and gelled particles are then washed and dried at arelatively low temperature of about 50°-200° C. and subjected to acalcination procedure at a temperature of about 450°-700° C. for aperiod of about 1 to about 20 hours. This treatment effects conversionof the hydrogel to the corresponding alumina matrix. In a preferredembodiment, the calcined composite is washed to remove any remainingalkali metal cations that may be present. The wash solution ispreferably an aqueous ammonium solution, most preferably containingabout 0.5% NH₃ in water. After washing at about 95° C., the composite isdried at about 110° C. See the teachings of U.S. Pat. No. 2,620,314 foradditional details.

The process of this invention is applicable to the isomerization ofisomerizable alkylaromatic hydrocarbons of the general formula:

    C.sub.6 H.sub.(6-n) R.sub.n

where n is an integer from 2 to 5 and R is CH₃, C₂ H₅, C₃ H₇, or C₄ H₉,in any combination and including all the isomers thereof. Suitablealkylaromatic hydrocarbons include, for example, ortho-xylene,meta-xylene, para-xylene, ethylbenzene, ethyltoluenes, thetrimethylbenzenes, the diethylbenzenes, the triethylbenzenes,methylpropylbenzenes, ethylpropylbenzenes, the diisopropylbenzenes, andmixtures thereof.

It is comtemplated that any aromatic C₈ mixture containing ethylbenzeneand xylene may be used as feed to the process of this invention.Generally, such mixture will have an ethylbenzene content in theapproximate range of 5 to 50 wt. %, an ortho-xylene content in theapproximate range of 0 to 35wt. %, a meta-xylene content in theapproximate range of 20 to 95 wt. % and a para-xylene content in theapproximate range of 0 to 15 wt. %. It is preferred that theaforementioned C₈ aromatics comprise a non-equilibrium mixture. The feedto the instant process, in addition to C₈ aromatics, may containnonaromatic hydrocarbons, i.e. naphthenes and paraffins in an amount upto 30 wt. %.

The alkylaromatic hydrocarbons for isomerization may be utilized asfound in selective fractions from various refinery petroleum streams,e.g., as individual components or as certain boiling range fractionsobtained by the selective fractionation and distillation ofcatalytically cracked gas oil. The process of this invention may beutilized for conversion of isomerizable aromatic hydrocarbons when theyare present in minor quantities in various streams. The isomerizablearomatic hydrocarbons which may be used in the process of this inventionneed not be concentrated. The process of this invention allows theisomerization of alkylaromatic containing streams such as reformate toproduce specified xylene isomers, particularly para-xylene, thusupgrading the reformate from its gasoline value to a high petrochemicalvalue.

According to the process of the present invention, an alkylaromatichydrocarbon charge stock, preferably in admixture with hydrogen, iscontacted with a catalyst of the type hereinabove described in analkylaromatic hydrocarbon isomerization zone. Contacting may be effectedusing the catalyst in a fixed bed system, a moving bed system, afluidized bed system, or in a batch-type operation. In view of thedanger of attrition loss of the valuable catalyst and of operationaladvantages, it is preferred to use a fixed bed system. In this system, ahydrogen-rich gas and the charge stock are preheated by suitable heatingmeans to the desired reaction temperature and then passed into anisomerization zone containing a fixed bed of catalyst. The conversionzone may be one or more separate reactors with suitable meanstherebetween to ensure that the desired isomerization temperature ismaintained at the entrance to each zone. It is to be noted that thereactants may be contacted with the catalyst bed in either upward,downward, or radial flow fashion, and that the reactants may be in theliquid phase, a mixed liquid-vapor phase, or a vapor phase whencontacted with the catalyst.

The process of this invention for isomerizing an isomerizablealkylaromatic hydrocarbon is preferably effected by contacting thealkylaromatic, in a reaction zone containing an isomerization catalystas hereinafter described, with a fixed catalyst bed by passing thehydrocarbon in a down-flow or radial flow fashion through the bed, whilemaintaining the zone at proper alkylaromatic isomerization conditionssuch as a temperature in the range from about 0°-600° C. or more, and apressure of about 101 kPu (abs) to about 10,340 kPa (ga) or more.Preferably, the operating temperature ranges from about 300°-500° C. andthe pressure ranges from about 69 to about 6,895 kPa(ga). Thehydrocarbon is passed, preferably, in admixture with hydrogen at ahydrogen/hydrocarbon mole ratio of about 0.5:1 to about 25:1 or more,and at a liquid hourly hydrocarbon space velocity of about 0.1 to about20 hr⁻¹ or more, most preferably at 0.5 to 10 hr⁻¹. Other inert diluentssuch as nitrogen, argon, etc., may be present.

The particular product recovery scheme employed is not deemed to becritical to the instant invention. Any recovery scheme known in the artmay be used. Typically, the reactor effluent will be condensed with thehydrogen and light hydrocarbon components removed therefrom by flashseparation. The condensed liquid product is then subject to afractionation procedure to further purify the desired liquid product. Insome instances, it may be desirable to recover certain product species,such as ortho-xylene, by selective fractionation. In most instances, theliquid xylene product is processed to selectively recover thepara-xylene isomer. Recovery of para-xylene can be performed bycrystallization methods or most preferably by selective adsorption usingcrystalline aluminosilicates.

The following examples are presented for purpose of illustration onlyand are not intended to limit the scope of the present invention.

EXAMPLES

The examples present test results obtained when catalysts of theinvention were evaluated in an isomerization process. The catalysts wereevaluated using a pilot plant flow reactor processing a non-equilibriumC₈ aromatic feed comprising 52.0 wt. % meta-xylene, 18.5 wt. %ortho-xylene, 0.1 wt. % oara-xylene, 21.3 wt. % ethylbenzene, and 0.1wt. % toluene, with the balance being nonaromatic hydrocarbons. Thisfeed was contacted with 100 cc of catalyst at a liquid hourly spacevelocity of 2, and a hydrogen/hydrocarbon mole ratio of 4. Reactorpressure and temperature were adjusted to cover a range of conversionvalues in order to develop the relationship between C₈ ring loss andapproach to xylene equilibrium (as determined by product para-xylene tototal xylene mole ratio). At the same time, at each temperature, thepressure was chosen to maintain a constant mole ratio of C₈ naphthenesto C₈ aromatics of approximately 0.06.

EXAMPLE I

Gallium-substituted pentasil zeolite having an x-ray diffractioncharacteristic of ZSM-12 was prepared for a catalyst of the inventiondesignated as Catalyst A from a template, sodium gallate, and a silicasource. The sodium gallate in solution was added slowly to a solution ofthe template, triethylmethylammonium bromide, in deionized water. Thesilica source, Ludox AS-40, then was added slowly to thetemplate/gallate solution. The weight ratio of Ludox-40 to template wasabout 3:1. Hydrothermal crystallization was effected under autogenerouspressure at about 150° C. for 20 days, and the solids obtained werefiltered, washed with deionized water and dried at 100° C.

A quantity of gallium-substituted pentasil zeolite having an X-raydiffraction pattern equivalent to that of ZSM-5 was prepared for acatalyst of the prior art designated as Catalyst B by adding a silicasource, Ludox HS-40, to an aqueous solution containing an organictemplate, tetrapropylammonium bromide. The weight ratio of silica totemplate was about 1:1. A solution of sodium gallate was added to thesilica and template mixture in an amount to give about 1.0 wt. % galliumbased on the finished zeolite. The resultant mixture was autoclaved atabout 150° C. for approximately 140 hours. The zeolite obtained waswashed, filtered and dried to yield a gallium-substituted pentasilzeolite containing approximately 1.3 wt. % Ga.

Catalysts A and B respectively, were prepared by combining a quantity ofeach of the above zeolites sufficient to provide a zeolite content of4.5 wt. % in the finished catalyst with gamma-alumina ground to lessthan 30 mesh, deionized water, concentrated HNO₃ and extrusion aids toform an extrudable mixture. The composites were extruded, air dried atabout 110° C. and then calcined at a temperature of about 650° C. Aftercalcination, each of the composites was washed with aqueous ammoniasolution, oven dried at 110° C., and reoxidized in dry air at about 565°C.

The extrudates next were impregnated with a solution of chloroplatinicacid containing 4 wt. % hydrochloric acid (based on the calcinedspheres) to yield a final platinum concentration as shown. Theimpregnated spheres were oxidized and chloride adjusted at 525° C.,reduced in molecular hydrogen at 565° C., then sulfided with hydrogensulfide at ambient temperature to a target sulfur level of 0.1 wt. %.The two extrudates had the following approximate analysis:

    ______________________________________                                                         Catalyst A                                                                            Catalyst B                                                            (Invention)                                                                           (Prior Art)                                          ______________________________________                                        Apparent bulk density, g/cc                                                                       0.453     0.485                                           Platinum, wt. %    0.38      0.35                                             Chloride, wt. %    0.70      0.52                                             Sulfur, wt. %      0.08      0.17                                             ______________________________________                                    

Note that the platinum contents were adjusted in relation to apparentbulk density to be essentially the same on a volume basis forexperimental comparison purposes.

EXAMPLE II

The performance to Catalysts A and B for isomerization of xylenes andethylbenzene was compared using the non-equilibrium C₈ aromatic feed andtest conditions described hereinabove.

FIG. 1 shows the conversion of ethylbenzene as a function of paraxylenein the total xylenes. Para-xylene in total xylenes represents theapproach to equilibrium para-xylene content, and is used as a measure ofxylene-isomerization severity. Ethylbenzene conversion is industriallyimportant because ethylbenzene typically comprises a significant portionof C₈ -aromatic streams. Due to the close boiling points of ethylbenzeneand the xylenes, it is very difficult and costly to remove ethylbenzeneby fractionation. However, it must be excluded in order to prevent itsbuild-up in the separation/isomerization loop described hereinbelow. Themost preferred method for removing ethylbenzene is isomerization topara-xylene. It is well known in the art that isomerization ofethylbenze to xylenes is more difficult than interconversion of thethree xylene isomers, and therefore activity for ethylbenzeneisomerization is a critical catalyst property. As is shown in FIG. 1,the ethylbenzene conversion of Catalyst A of the invention issignificantly greater, at any level of xylene-isomerization severity,than of Catalyst B of the prior art.

At constant conversion, data indicate that the C₈ ring loss for CatalystA is 0.5-1 mole-% higher than for Catalyst B. This may be offsetindustrially by reducing the levels of ethylbenzene conversion andxylene isosmerization severity to limit the C₈ ring loss. Thus, for acomplete evaluation of catalyst performance, the effects of ethylbenzeneconversion, xylene isomerization severity, and C₈ ring loss on thecombined para-xylene separation and isomerization processes should beconsidered. This can be done by relating the recycle ratio andpara-xylene yield, as defined in the following.

In the separation/isomerization process combination, fresh C₈ -aromaticfeed is combined with C₈ -aromatics and naphthenes from theisomerization reaction zone and fed to the para-xylene separation zone;the para-xylene depleted stream is fed to the isomerization reactionzone, where the C₈ -aromatic isomers are again isomerized tonear-equilibrium levels. In this process scheme the C₈ -aromatic isomersare recycled to extinction, until they are either converted topara-xylene or lost due to side-reactions. The presence of a C₈-naphthene stream throughout the process is necessary to provide theintermediates required for ethylbenzene isomerization in theisomerization reaction zone. The para-xylene yield for the process isdefined as the weight fraction of the fresh C₈ -aromatic feed that isultimately converted to the para-xylene product stream. The recycleratio is defined as the weight ratio of rate of recycle of C₈-aromatic+naphthenes from the isomerization reaction zone to the rate offresh C₈ -aromatic feed addition to the process. In a graph ofpara-xylene yield versus recycle ratio, superior catalyst performance isreadily distinguished as superior para-xylene yield at a given recycleratio. Industrially, operating conditions are a balance betweendesirable high para-xylene yield and undersirable utility cost thatincrease with recycle ratio.

The results for Catalysts A and B are shown in FIG. 2. It is evidentfrom the Figure that at low recycle ratios, at which utility costs areminimized, Catalyst A of the invention provides a higher para-xyleneyield than the prior art Catalyst B. Stated differently, thesecalculations show that at low recycle ratios, the high ethylbenzeneconversion of the catalyst of this invention outweights the increase inC₈ ring loss at constant conversion over the prior art Catalyst B, thusproviding an improved overall process.

We claim:
 1. A process for the isomerization of a non-equilibrium feedmixture of xylenes containing ethylbenzene comprising contacting thefeed mixture in the presence of hydrogen at isomerization processconditions with a catalyst comprising at least one Group VIII metalcomponent and a gallium-substituted pentasil zeolite having an x-raydiffraction pattern characteristic of ZSM-12.
 2. The process of claim 1wherein the isomerization process conditions comprise a temperature offrom about 300° to about 500° C., a pressure of from about 69 to about6895 kPa (ga), a liquid hourly space velocity of from about 0.5 to 10hr⁻¹ and a hydrogen to hydrocarbon mole ratio of about 0.5:1 to about25:1.